If you've never been to CN and are considering it for a future visit then it's really worth it. The water is clearer and smoother at CN than CSA, not as long but deeper. We enjoyed having lunch at the Cassava Terrace as a change up for that offered at CSA.
I think it might depend on how long you are staying at CSA. If you are staying less than a week, you might not want to take a day away from the resort. We did the trading spaces to CN, and thought it was worthwhile. It was nice to have a change of pace for the day. Same great service at CN, but a different view.
We are doing trading places this year, 4th CSA trip and have not done this before, but really looking forward to it. Stayed at RPTB four years ago so we know the beach and general area around Bloody Bay, even walked about Couples Negil a bit as well, so very familiar with the area. We almost picked Couples Negril this year, but just couldn't part with our stay at CSA. Need more vaca time to do both!!
Definitely worth it!! The beach is smaller than CSA but still walkable. Plus you can stop by the Office of Nature (north on the beach) for some lobster. Definitely check it out for the afternoon - and if you want to head back to CSA sooner, they will take you back. We did it last year from noon to 3p (had to wait until hubby got back from his CSA morning dive).